This invention relates to an electronic analog switching device, comprising an input terminal, an output terminal, a differential voltage amplifier having an inverting input, a non-inverting input and an output, a first on-off switch connected between the amplifier output and the output terminal, a feedback circuit from the output of the amplifier to its inverting input, and control means for controlling the on and off states of the first switch, the input terminal being connected to the non-inverting input of the amplifier.
Such a device is well known and is used, for example, as the analog switch of a sample-and-hold circuit in which an analog voltage signal is sampled at regular instants and the sampled voltage at each instant is stored, between successive instants, on a capacitor connected to the output terminal of the device. The impedance presented to the voltage signal by the sample-and-hold circuit must, of course, be high relative to that of the signal source and must also be constant. On the other hand, the impedance of the source which feeds the voltage sample to the storage capacitor must be as low as possible in order to achieve a very rapid charging of the capacitor. For these reasons, the signal voltage source is permanently connected to an operational amplifier, that is to say a high gain stable d.c. amplifier which has a high input impedance and a low output impedance, and the amplifier output is switched at the sampling instants to the capacitor. In order that the sampled voltage, sampled by the switch, is the same as that from the signal source at that instant, the gain of the operational amplifier is made unity by using a differential amplifier, having an inverting and a non-inverting input, with a feedback path from its output to its inverting input, the input signal being fed to its non-inverting input.
In the known analog switching device of the type described above, not only must the amplifier have a low output impedance but also the switch must have a very low "on" state impedance in order to minimize the effect of any output load (such as the capacitor in the above-mentioned sample-and-hold circuit) on the operation of the device. In a sample-and-hold circuit, the capacitor is normally followed by a further high impedance buffer amplifier and it is important to minimize leakage in the switching device; for example to minimize any discharge of the capacitor between sampling instants. The effect of leakage can be reduced by using a large capacitance value but then, for a given charging time for the capacitor, the series impedance of the switch in the "on" state (its "on" impedance) and the output impedance of the operational amplifier must be made proportionally lower. In integrated circuit form, this would mean that the area taken up by the switch and the amplifier would have to be relatively large which is a considerable disadvantage not only in size but also in cost. Further, the increase in area of the switch results in greater leakage and, hence, in reduced circuit performance.
It is also possible for an unwanted voltage transient to be generated the instant the switch is turned on. Also, in some circumstances, there may in practice by cross-talk, or cross-coupling, between the two inputs (inverting and non-inverting) of the operational amplifier.
The circumstances which lead, or may lead, to the above-mentioned disadvantages are explained below in more detail.